PPTP (Point-to-Point Tunneling Protocol) is a network protocol designed to implement virtual private networks (VPNs) by creating a secure tunnel for data transmission between a remote client and a server over the internet. Developed in the 1990s by a consortium led by Microsoft, PPTP was one of the earliest protocols used for VPNs and played a significant role in enabling private, remote access to networks.
PPTP establishes a tunnel between a VPN client and a VPN server by encapsulating data packets within a Point-to-Point Protocol (PPP) frame, which is then further wrapped in a Generic Routing Encapsulation (GRE) header. It uses TCP port 1723 to set up a control channel for managing VPN sessions, while the actual data transmission occurs over the GRE tunnel. Authentication is typically handled by protocols like MS-CHAPv2, and data encryption is provided by Microsoft Point-to-Point Encryption (MPPE), which uses the RC4 stream cipher. PPTP supports both voluntary (client-initiated) and compulsory (server-initiated) tunneling modes.